1. Field of the Invention
The present invention relates to a charged particle beam extraction system and method for extracting an ion beam, e.g., a proton or carbon ion beam, through an irradiation device.
2. Description of the Related Art
There is known a therapy method for irradiating an ion beam, e.g., a proton or carbon ion beam, to an affected part in the body of a patient, such as a cancer. A particle beam therapy system for use in such therapy comprises an ion beam generator, a beam line, and an irradiation device of the rotation type, for example. An ion beam accelerated by the ion beam generator reaches the irradiation device through a first beam line, and is irradiated to the affected part in the patient body from an irradiation nozzle having passed through a second beam line installed in the irradiation device. Known examples of the ion beam generator include means for circulating the ion beam along an orbit, means for bringing betatron oscillation of the ion beam into a resonant state outside the separatrix of resonance, and a circular accelerator provided with an extraction deflector for extracting the ion beam from the orbit (see, e.g., Patent Reference 1; U.S. Pat. No. 5,363,008).
The therapy using an ion beam, e.g., the treatment with irradiation of a proton beam, is based on characteristics that most of energy of the proton beam is released at the time when protons are stopped (to provide a Bragg peak). Then, the energy of the proton beam is selected to stop protons near the affected part in the patient body so that most of the energy (absorbed dose) is given only to cells of the affected part.
Usually, the affected part is present in some organ and therefore has a certain thickness in the direction of depth from the body surface of a patient (i.e., the direction of travel of the ion beam). To effectively irradiate the ion beam over the entire region of the affected part in the direction of thickness, the ion beam must be controlled so as to form a comparatively wide and flat range of absorbed dose in the direction of thickness (i.e., a spread-out Bragg peak, hereinafter referred to as an “SOBP”).
From that point of view, a ridge filter has hitherto been proposed which includes a plurality of structures each shaped to have a thickness increasing or decreasing step by step and arranged cyclically in a direction perpendicular to the direction of travel of the ion beam, thereby changing the thickness of the filter, through which the ion beam passes, depending on the beam incident position (see, e.g., Non-Patent Document 1; “REVIEW OF SCIENTIFIC INSTRUMENTS”, Vol. 64, No. 8 (August 1993), p. 2078 and FIG. 31). The ion beam is irradiated to the affected part in the patient body through the ridge filter. More specifically, when the ion beam passes an opening between the adjacent structures, the ion beam passes through the ridge filter without attenuating its energy and the Bragg peak is produced in a deep position inside the body. When the ion beam passes a thin step portion of the structure, the beam energy is slightly attenuated and the Bragg peak is produced in a middle position inside the body. When the ion beam passes a thick step portion of the structure, the beam energy is largely attenuated and the Bragg peak is produced in a shallow position near the body surface. Since the Bragg peaks are thus produced at different depths in the patient body, the SOBP can be obtained over a comparatively wide region from the position near the body surface to the position deep inside the body.
As another means for producing the SOBP, a range modulation wheel (hereinafter abbreviated to “RMW”) has already been proposed which includes a plurality of blades each having a thickness increasing or decreasing step by step in the circumferential direction (see, e.g., Non-Patent Document 2; “REVIEW OF SCIENTIFIC INSTRUMENTS”, Vol. 64, No. 8 (August 1993), p. 2077 and FIG. 30). The RMW is set in the travel path of the ion beam and is rotated in a plane perpendicular to the direction of travel of the ion beam. At the time when the ion beam enters an opening between the adjacent blades, the ion beam passes through the RMW without attenuating its energy and the Bragg peak is produced in a deep position inside the body. At the time when the ion beam passes a thin step portion of the blade, the beam energy is slightly attenuated and the Bragg peak is produced in a middle position inside the body. At the time when the ion beam passes a thick step portion of the blade, the beam energy is largely attenuated and the Bragg peak is produced in a shallow position near the body surface. With the rotation of the RMW, the position of the Bragg peak varies cyclically. As a result, the SOBP can be obtained over a comparatively wide region from the position near the body surface to the position deep inside the body, looking at the beam energy integrated over time.
On the other hand, as one irradiation method for improving a degree of matching of dose distribution with the shape of the affected part and minimizing useless dose irradiated to the surrounding organs, there is known a pencil-beam scanning method of scanning a pencil-shaped small-diameter beam in match with the shape of the affected part. Regarding an irradiation technique for use with the pencil beam scanning method, it is proposed to divide an irradiation area into small targets (referred to as “spots” hereinafter), to stop the beam when the beam has been irradiated in preset dose to one spot, and to start a next cycle of irradiation as soon as after preparations for the irradiation to the next spot have been finished, followed by repeating the steps of stopping the beam when the beam has been irradiated in preset dose to the next spot and making preparations for the irradiation to the subsequent spot again (see, e.g., Patent Document 2; Japanese Patent No. 2833602). In that case, when a circular accelerator is used, control in the depth direction is performed by successively changing the setting of target energy. More specifically, the affected part in the patient body is divided into a plurality of layers in the direction of depth of the affected part. After the irradiation for all spots in each of the layers is completed, the target energy setting of the circular accelerator is changed and the irradiation is shifted to another layer. In the same layer, the beam is successively irradiated to individual spots by repeating the start and stop of the beam irradiation at the same energy.